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Use Cucumber Table Transformations To Build Objects

Using Cucumber’s Table Transformations, you can easily build complex objects in a way that’s easy to read and understand for clients and developers alike.

My latest favorite feature of Cucumber are Table Transformations. I frequently use tables to build up complex objects and I’ve found that the regular old tables can be a little ugly, especially when your attribute names don’t make much sense on their own. I’ve also noticed that building up associations can be a little wonky, usually requiring more steps than seem necessary.

Conventional Table Usage

Let’s look at an example of how we could use a table, without transformations, to build up some objects for our scenario.

Given /^I have a spirit with the following attributes:$/ do |table|
table.hashes.each do |attributes|
Spirit.create!(attributes)
end
end

Be sure to use create! in your tests to prevent false positives (Thanks Aslak!)

Now this is all fairly simple, and it looks pretty easy to implement, but I see some problems. First, what if you were actually friends with your DBA (bear with me) and you knew better than to have an attribute in your model like country_of_origin or spirit_type. Chances are those are going to be used by many other records and should be pulled out and made into their own Models, Country and SpiritType respectively.1

So what does our scenario look like with those two new models?

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Scenario: Editing a Spirit
Given I have a country with the following attributes:
| name | continent |
| Scotland | Europe |
And I have a spirit type with the following attributes:
| name |
| Scotch Whiskey |
And I have a spirit with the following attributes:
| age | brand | lgcy_prod_sku | name |
| 12 | The Balvenie | SC38181 | DoubleWood |
| 12 | The Macallan | SC38245 | |

It’s a little more complex, for sure, but it’s not totally unmanageable. However, the key part that’s missing is how to link the two spirits with their spirit types and countries of origin.

You could add some more steps, but then you’ve got a conjunction step which is inflexible and brittle.

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And the spirit named "The Balvenie" is from "Scotland" and is a "Scotch Whiskey"

You could go back to your original step, and try to do some behind-the-scenes stuff to map country_of_origin to the correct country_id, but that gets messy too.

Transform Your Tables

The first step to making good use of table transformations is to make your tables more readable. Start by change the header row of your table to use meaningful representations of the real attribute names.

We’ve turned that weird lgcy_prod_sku attribute into something that your Product Owner can make sense of and we’ve be able to add Spirit Type and Country of Origin back to the table. Now let’s look at the transformation that makes this all work.

The transformation step definition looks a lot like a regular table step definition. There is a regular expression, like anything else in Cucumber, that has the same values as the header row in the table from our scenario. Just like the table step definition we have a table object which is just an array of hashes. We can go through each hash, do the actual transformation, and then return something to our table step definition. We are using map (same as collect) to return an array of hashes, which is just what the table step definition is expecting.

You will also see that we’re creating three different records, which we are returning in the hash we create at the end. Let’s go through those step-by-step:

Create a spirit type object from the hash["Spirit Type"] value

Create a country object from the hash["Country"] value

Create a spirit object from the several related hash values

Put all of our created objects into a hash

While we’ve created the objects, we still need to create the associations. I like to leave this for the table step definition rather than the transformation since I think it’s more obvious what’s going on with the values when you’re viewing the table step.

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Given /^I have a spirit with the following attributes:$/ do |table|
table.each do |group|
spirit = group[:sprit]
associations = {:country => group[:country], :spirit_type => group[:spirit_type]}
spirit.update_attributes(associations)
end
end

There are at least a dozen ways to get the spirit associated with the country and spirit type, so don’t feel like you have to follow this pattern every time. Since we’ve sent our table an array of hashes we can iterate over each hash, group, and work with the individual rows. Here’s how:

Extract the spirit object from the hash

Create another hash with the country and spirit type that Rails can make sense of

Use update_attributes to update the spirit object with the new associations

Transformation Tradeoffs

We’ve been able to take our original multi-step scenario and simplify it to a single step. We are using the proper place, the step definitions, to do the associations and we have made our scenario much easier to read for non-developers working on the project. But what did we give up?

The biggest issue I’ve found with using table transformations is that they can be inflexible when you need to add more attributes to your dynamically created object. If you are writing features, using your table to setup objects and then realize that you need to add another attribute, you’re going to have to edit your table transformation step and how you create objects from the hash. When you take this a step further and try to have two different table definitions, you’ll be looking at having two nearly identical table transformations.2

If you’re not already using regular old Cucumber tables to create objects, use this guide to get started. If you are using Cucumber tables to create objects, try to re-factor one of your scenarios and use the table transformation strategy. Once you start using Cucumber tables and table transformations you’ll instantly improve the readability, portability and efficiency of your steps.

1 Ignore for now the issues with spirit_type and Rails Single Table Inheritance

2 I’m guessing that there is some way you can get around this with regular expressions and to have more flexible transformation table steps, but I haven’t tried it yet.

I did not know about Spirit.create!(table.hashes) but now that I look at it it makes perfect sense. Thanks for the tip on create! I was experiencing that problem today when I deviated from using factory_girl Factories.

I’m not sure how you can get cucumber to preserve whitespace in the table definition. Have you tried putting an actual string inside the table definition " shilpa " and then reformatting that in your step?

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